Semiconductor wafers are typically fabricated with multiple copies of a desired integrated circuit design that will later be separated and made into individual chips. A common technique for forming the circuitry on a semiconductor is photolithography. Part of the photolithography process requires that a special camera focus on the wafer to project an image of the circuit on the wafer. The ability of the camera to focus on the surface of the wafer is often adversely affected by inconsistencies or unevenness in the wafer surface. This sensitivity is accentuated with the current drive toward smaller, more highly integrated circuit designs. Semiconductor wafers are also commonly constructed in layers, where a portion of a circuit is created on a first level and conductive vias are made to connect up to the next level of the circuit. After each layer of the circuit is etched on the wafer, a dielectric layer is put down allowing the vias to pass through but covering the rest of the previous circuit level. Each layer of the circuit can create or add unevenness to the wafer that is preferably smoothed out before generating the next circuit layer.
Chemical mechanical planarization (CMP) techniques are used to planarize the raw wafer and each layer of material added thereafter. Available CMP systems, commonly called wafer polishers, often use a rotating wafer holder that brings the wafer into contact with a polishing pad moving in the plane of the wafer surface to be planarized. A polishing fluid, such as a chemical polishing agent or slurry containing microabrasives, is applied to the polishing pad to polish the wafer. The wafer holder then presses the wafer against the rotating polishing pad and is rotated to polish and planarize the wafer.
During the polishing process, the properties of the polishing pad can change. Slurry particles and polishing byproducts accumulate on the surface of the pad. Polishing byproducts and morphology changes on the pad surface affect the properties of the polishing pad and cause the polishing pad to suffer from a reduction in both its polishing rate and performance uniformity. To maintain a consistent pad surface, provide microchannels for slurry transport, and remove debris or byproducts generated during the CMP process, polishing pads are typically conditioned. Pad conditioning restores the polishing pad's properties by re-abrading or otherwise restoring the surface of the polishing pad. This conditioning process enables the pad to maintain a stable removal rate while polishing a substrate or planarizing a deposited layer and lessens the impact of pad degradation on the quality of the polished substrate.
Typically, during the conditioning process, a conditioner used to recondition the polishing pad's surface comes into contact with the pad and re-abrades the pad's surface. The type of conditioner used depends on the pad type. For example, hard polishing pads, typically constructed of synthetic polymers such as polyurethane, require the conditioner to be made of a very hard material, such as diamond, serrated steel, or ceramic bits, to condition the pad. Intermediate polishing pads with extended fibers require a softer material, often a brush with stiff bristles, to condition the pad. Meanwhile, soft polishing pads, such as those made of felt, are best conditioned by a soft bristle brush or a pressurized spray.
One method used for conditioning a polishing pad uses a rotary disk embedded with diamond particles to roughen the surface of the polishing pad. Typically, the disk is brought against the polishing pad and rotated about an axis perpendicular to the polishing pad while the polishing pad is rotated. The diamond-coated disks produce predetermined microgrooves on the surface of the polishing pad. Another method used for conditioning a polishing pad uses a rotatable bar on the end of a mechanical arm. The bar may have diamond grit embedded in it or high pressure nozzles disposed along its length. In operation, the mechanical arm swings the bar out over the rotating polishing pad and the bar is rotated about an axis perpendicular to the polishing pad in order to score the polishing pad, or spray pressurized liquid on the polishing pad, in a concentric pattern.
The life of a polishing pad is a key factor in the cost of a CMP process. By applying abrasive materials directly to the surface of the polishing pad, conventional pad conditioners, as described above, erode the surface and reduce the life of the polishing pad. Accordingly, advances in methods and apparatuses for conditioning polishing pads used in the chemical mechanical planarization of semiconductor wafers, are necessary to improve, for example, polishing pad life.